Various methods for the production of large single crystals have been proposed and described, particularly in the silicon art. Interest in this development has been stimulated to a large degree by expanded interest in solar cells, the most common of which are made from silicon. While the raw material is inexpensive, efficiency of energy conversion depends on crystal size in a polycrystalline silicon film, and it has been difficult and expensive to find methods for cheaply and efficiently producing large areas of high purity single crystal material.
Presently, commercial processes for the production of high purity silicon include zinc reduction of silicon tetrachloride, and thermal decomposition of trichlorosilane at slow rate followed by Czochralski crystal growth. Various other processes like zone refining, dendrite web processes or edge-defined, film-fed growth are further examples of other methods which are being or have been used to produce single-crystal silicon in the semiconductor field. At the present time, the most promising of these methods appears to be the use of chemical vapor deposition methods using metallurgic grade silicon or a silicon dipping process, whereby the silicon is deposited upon a wettable substrate. Partial grain enhancement of the films may then be achieved as by laser or moving strip heater recrystallization. It is also known that crystal growth can be enhanced if an artificial pattern is etched on the substrate before thermal annealing, see "Silicon Grapheopitaxy Using a Strip Heater Oven" by M. W. Geis, et al, Appl. Phys. Lett 37(5) (1980) pp. 454-456. For example, the pattern may be a multiplicity of closely spaced parallel lines. Subsequent heat treatment by the use of either a laser or an oven may be used to effect controlled recrystallization in order to promote larger crystal size. These methods realize thin film polycrystalline silicon solar cells exhibiting efficiencies of between 9.5 to 10.1 percent.
The use of ultra sound to induce physical movement of solid iron oxide crystals to orient tnem in parallel relation in order to materially increase the sensitivity of magnetic tapes is disclosed in U.S. Pat. No. 3,194,640. Directional solidification of fine grain semiconductor bodies of chalcogenides under the influence of controlled sonic vibration is disclosed in U.S. Pat. No. 3,867,496. The purpose of the sonic energy is not entirely clear from this patent, although it is clear that there is no suggestion of forming a standing wave.